Forming the lifeline between Earth and her 20-plus colonized worlds are the thousands of starships manned by over a hundred thousand crew members. Along with the hundreds of thousands of space settlers that live and work aboard orbital terminals, habitats, and other space facilities, this large pool of people have had to learn to work against and cope with their zero-g environment.
The basic physiological effects of zero gravity on the human body have been studied for over 300 years. Compiling and analyzing tons of data, scientists have been able to prove what they have believed for centuries. That is simply that the presence of gravity is required for normal, healthy human development. The most obvious zero-g related health problem -- motion sickness -- a problem easily countered by the myriads of commercially available modern day medications. Any of which should work perfectly well. The second is the loss of muscle mass. Without the constant pull of gravity to work against, many muscles become underutilized and weakened. Various solutions have been attempted in order to counter this problem including the use of electric current or genetically engineered products to stimulate muscle growth. While these have worked to some extent, a regular exercise program has proven to be the best method for reducing muscle loss and for countering the many other related health effects. Besides muscle, the human body also loses bone mass in zero-g. Fortunately for the future of human colonization of space, this effect was counterable through the use of certain proteins made available in the early 21st century. Such medication is regularly administered though shipboard food supplies.
"For every action, there is an equal and opposite reaction"
In zero-g if you exert a force on an object, say by pushing it away from you, the same force is applied to you and you are pushed back in the opposite direction. Small objects don't take much force to push, so you don't notice the same force being applied on you. If you push a large object, or a small one very quickly, you will find yourself pushed back to a much greater degree. This is especially noticeable in the handling of firearms where a small but very fast bullet is pushed out of a gun barrel while the gun is pushing back on you with the same amount of force. In the case of a massive object, you will find it nearly impossibly to move without getting some leverage off of a handhold, wall, or more massive object.
"An object in motion remains in motion unless acted on by an outside force"
Inertia can be a dangerous thing. Without gravity, there is generally no friction to naturally slow the movement of heavy objects. So, whatever energy is used to move an object, the exact amount must be used to stop its motion. The problem is that if one person pushes an object as hard as he can for 10 seconds, he would have to push it as hard as he could for 10 seconds in order to stop it. Two people can stop it in 5 seconds, and so on. The problem is that if a person doesn't pay attention to what he or she is doing, something or someone can get crushed fairly easily.
The immediate problem of adapting to life in zero-g is the simple loss of the one thing that people rely on so much without really realizing it -- the pull of gravity. For those unaccustomed to the absence of gravity, the first experience can be enjoyable, but can quickly become frustrating. It is interesting how much people rely on gravity and its effects. Adapting to zero-g has resulted in a very different kind of living environment. First off, you can't sit, but then you really don't need to. Most shipboard workstations are equipped with acceleration couches, but for ships which are not landing capable, these tend to be more of a contoured padded leaning post that you strap yourself to. Most other 'chairs' consist of a pad that you lean on and padded bar to lock your legs into to keep you from drifting away. Next are sleeping accommodations. These usually consist of a padded mattress, much like a normal bed. The main difference is that you either strap yourself in to keep from 'sleep floating' or you zip yourself in. Aboard ships with a minimum of crew space, particularly non-landing capable ones, the berths may be mounted vertically on a wall to take up less room. Sinks and toilets are among the most difficult development problems that mankind has had to face. Numerous systems have been developed, and are in use. None of them are either graceful or pleasant -- most are rather awkward and embarrassing. Toilets are one of the most appreciated luxuries of living in a gravity or spin environment.
For those looking for more role-playing specific information about zero-g environments, GDW's Nyotekundu module is your best ticket. It describes not only zero-g operations in game terms, but also describes the effects of spin. In any case, zero-g and spin are difficult environments to adventure in. If it's just too heavy for you and/or your players, just keep it simple. Don't let it hamper your enjoyment of the game.